Progressive deformation method and means using tubular blanks



Oct. 28, 1969 J. s. CORRAL ETAL 3,474,649

PROGRESSIVE DEFORMATION METHOD AND MEANS USING TUBULAR BLANKS Filed Nov.30, 1966 INVENTORS DONALD E. KRANTZ JOSEPH S. CORRAL K M-k- ATTORNEYUnited States Patent O m US. CI. 7256 ABSTRACT OF THE DISCLOSURE Thedisclosure in this case describes essentially a twostep process ofexplosively forming bulbous articles, one form of which isillustratively described as spherical. The process begins by explosivelyswaging a hollow tubular blank to reduce the diameter thereof in atleast two spaced-apart locations along the tube length by wrapping eacnstated location with an explosive and detonating the same. Thereafter,the final article shape is achieved by centering an explosive chargewithin the tubular workpiece equidistant from the two swaged neckportions and detonating the same to expand the workpiece radiallyoutwardly. The expansion step contains further novelty in that a diecavity is evacuated by surrounding the die with a vacuum chamber. Thetwo basic steps when used succe sively on the same workpiece permit highdiametral expansion to be achieved in articles having length greaterthan critical length without the occurrence of local thinning orballooning in the expanded portion. In addition, the swaging step isaccomplished without any tooling or items of apparatus being required.

BACKGROUND The invention disclosed herein is applicable to thefabrication of any article which can be formed by expansion of agenerally tubular blank by the application of pressure from within thesame. The method disclosed herein avoids ballooning or excessivethinning of metal in localized areas during expansion of the blank by apreliminary step of compressing the blank to reduce the amount oflocalized expansion occurring during the subsequent expansion step.

One result of the method disclosed herein is that an increase indiameter up to 100 percent greater than the initial starting diameter ofthe tubular blank may be achieved with nearly uniform wall thicknessthroughout the final article. Moreover, the method disclosed herein isparticularly useful in forming unitary articles of such diflicult shapesas spherical tanks or containers characterized by complete absence ofjoining seams or welds throughout the surface of the final article. Incontrast to the foregoing result, conventional methods for expandingtubular blanks into articles of spherical or bulbous form are successfulprimarily in the case of very small diametral expansion ratios only, theprecise value of which depends upon the initial length of the blankconsidered with the starting and final diameters of the workpiece beforeand after deformation of the same to produce the finished article.Specifically, the amount which a workpiece of any given metal or alloyin tube or cylindrical form may be successfully expanded by conventionaltube forming devices without resulting in rupture of the workpiecematerials is partly dependent upon the length of the workpiece overwhich the expansion occurs. For example, a ten percent increase indiameter of a tubular workpiece of short length may be achieved-withrelative ease by applying fluid pressure within the workpiece suificientto force the sides radially outwardly in all directions an equal amount,whereas the same diametral ratio of expansion could not be. accomplishedby the stated method Patented Oct. 28, 1969 ice in a workpiece ofrelatively great length. In the longer workpiece, application of forcefrom within the tube in an amount sufficient to exceed the yieldstrength of the material therein will result in ballooning or localthinning of the workpiece at some area along its length, followed byrupture of the workpiece material at the stated area if the applicationof radially outward force is continued. Between the two stated extremesthere lies a particular value of length for any given workpiece size,material, condition and expansion ratio which may be termed the criticallength, at which expansion under the stated conditions may occur withoutrupture, and which if exceeded will result in localized anduncontrollable non-uniform thinning followed by rupture. For example,uniform expansion of a tubular blank of aluminum to a diameter in excessof five percent by conventional means when the critical length isexceeded has been found to result in rupture of the material even withsuperior uniformly of material composition, thickness and surfacesmoothness in the blank before the forming operation is begun.

OBI ECT S Accordingly, it is a principal object of the invention in thiscase to provide improved method and means for forming tubular blanksinto articles of bulbous form having larger area at one or morecross-sections therethrough than the stated blank.

It is also an object of this invention to provide improved method forswaging tubular workpieces to form throat sections or the like havingreduced diameter and increased wall thickness, by means avoidingapplication of force to such workpieces by dies or the like.

It is another object in this case to provide method and means forexpanding tubular blanks into various shapes by pressure appliedoutwardly by explosive force from within the tube walls.

It is another object of this invention to provide method and meanspermitting expansion of tubular blanks to form articles having lengthgreater than critical length and cross-sectional sizes substantially inexcess of the cross-sectional size of such blanks.

It is a further object of the invention in this case to provide methodand means as set forth in the above objects having improved ease,rapidity and lower cost than known methods of producing bulbousarticles.

Other objects and advantages will become apparent upon a close readingof thef ollowing detailed description of an illustrative embodiment ofthe inventive concept, reference being had to accompanying drawings,wherein:

FIGURE 1 shows a cross-sectional view taken along the longitudinalcenter line of an elongate tubular blank arranged for the application ofexplosive force thereto as suggested by line 11 in FIGURE 2,

FIGURE 2 is an end elevational view of the structure shown in FIGURE 1,

FIGURE 3 shows a workpiece deformed by use of the novel method depictedin FIGURES l and 2,

FIGURE 4 is a side elevational View, partly broken away and in crosssection to reveal details of the apparatus containing a portion of theworkpiece from FIGURE 3 and arranged for a final forming step accordingto the novel method disclosed herein, and

FIGURE 5 is an isolated end elevational view of a portion of theapparatus shown in FIGURE 4.

Referring to the drawings described above, it may be seen that theinventive concept disclosed herein contemplates and includes a workpiecewhich is initially of tubular form as shown by tube 10 in FIGURE 1, andis formed by a succession of steps according to the novel methodsdisclosed herein into an article of desired final shape. Illustratively,the final article may be generally spherical corresponding to the dieworkface contour shown in FIGURE 4. The forming process begins with theinitail compression step suggested by FIGURES 1 and 2, wherein it may beseen that tubular blank is filled with a force absorbing or crushablefiller material 12 such as foamed in-place plastic, rubber or plaster.Alternatively, filler 12 may comprise water, provided that the ends oftube 10 (not shown) are first plugged so that the tube will becompletely filled without ullage spaces.

Filler 12 functions to prevent lopsided buckling of tube 10 uponapplication of explosive force symmetrically about the outer surfacethereof, in case of workpiece blanks having relatively thin walls.Localized buckling of workpiece 10 is unlikely where the initialthickness of the tubular blank, designated 14 in FIGURE 1, issubstantial such as one-fourth inch with a starting diameter of fourinches or less, whereby the workpiece material is sufiiciently rigid toabsorb and safely distribute the entire compressive force, and filler 12may be safely omitted.

When filler 12 is used, and is contained within workpiece 10 so as tocompletely fill the same, the workpiece portion which is sought to bedeformed by the novel method suggested in FIGURE 1 is wrapped with alayer or sleeve of resilient-force-distributing material 16. Also,sleeve 16 protects the workpiecefrom burning effects of the explosive,and may comprise a relatively thick sheet of hard rubber or the like.Sleeve 16 is concentric with and in substantially continuous contactwith the outer surface of tube 10 throughout the length of the sleeve.Thereafter, one or more layers of explosive in sheet form are wrappedabout tube 10 on the radially outer surface of sleeve 16 in the mannersuggested by the three layers 18 or explosive shown in FIGURE 1 wherebythe explosive and the sleeve are concentric and co-axially related tothe longitudinal axis of elongate tube 10. Thereafter, suitableexplosive detonating means which may take the form of electricallyenergized blasting caps 20 are preferably arranged substantiallysymmetrically about items 10, 16 and 18 as shown more particularly byFIGURE 2.

It will be understood that several different kinds of explosive known tothe prior art may be used to generate the explosive force capable ofdeforming tube 10 in the manner suggested by FIGURE 1, such as anexplosive in cord form wound about the blanket 16, and that the amountof explosive used will vary according to the rigidity and thickness ofthe material comprising tubular blank 10. The purpose of using aplurality of electrically .detonated blasting caps 20 symmetricallyarranged about the workpiece and suggested by FIGURE 2 is to achieve asnearly as possible the simultaneous detonation of the circularlyarranged explosive charge. The force generated by detonation of theexplosive applies a substantially continuous compressive load abouttubular blank 10 deforming the same and reducing the diameter thereof assuggested by broken lines 22 shown in FIGURE 1 and solid lines 24, 26and 28 in FIGURE 3. The foregoing reduction in diameter is accompaniedby a substantial increase in the wall thickness of tubular blank 10 inthat portion which is compressed by explosive force in the mannerdescribed above. Moreover, the process thus described and shown byFIGURE 1 may be repeated one or more times at the same location toproduce successive reductions of workpiece diameter until a desiredfinal value thereof is achieved. The amount of diameter reductionobtainable with each individual application of force depends upon theinitial wall thickness, the initial tube diameter, the workpiecematerial in the tubular blank 10, and the kind of explosive used, all ofwhich may be determined according to well-known principles in theexplosive forming art. In addition, a greater amount of wall thicknessincrease may be achieved for a given amount of diametral decrease whereblank 10 is relatively short and thin walled initially, by restrainingboth ends of the workpiece against movement as suggested by fixedstructural surface 23 hearing against the end of blank 10 as shown inFIGURE 1, it being understood that, where necessary or desirable, bothends of the blank are so restrained to prevent elongation of theworkpiece simultaneous with diametral reduction thereof.

The initial step described above in practicing the novel methodsdisclosed herein may be'conveniently termed explosive swaging, andrepresents certain definite improvements over conventional swagingmethods which involve mechanical devices for applying force locally andprogressively about a tube to reduce the diameter thereof such as byrotary swaging dies. Thus, the forces which may be applied mechanicallyare severely limited by the strength of the die materials and otherfactors, including the need for frequent replacement of such dies due toexcessive wear resulting from their use with workpiece materials of highstrength steel or the like. The method described above and suggested byFIGURE 1, for example, results in improved accuracy in the surfacecontour, symmetry and uniformity of workpiece thickness and is extremelyversatile with regard to the workpiece materials which may besuccessfully swaged and the high percent of diametral decreaseachievable by the foregoing method. Moreover, the stated advantages areachieved without any tooling being required. Thus, the stated method hasbeen successfully used to effect diametral reductions of 50 percent andhigher in workpiece blanks of stainless steel, using three successivedetonation steps identical to that suggested in FIGURE 1.

The explosive swaging technique described above has been found extremelyuseful in the preparation of workpiece tubular blanks to form bulbousshapes such as spheres and the like, wherein swaging is accomplished atspaced-apart locations along an elongate tubular blank 10 illustrativelyshown by FIGURE 3. In the stated figure, swaging by the method describedabove at a plurality of locations along workpiece 10 results in reducedworkpiece diameters indicated by reference numerals 24, 26 and 28, withintermittent bulbous portions 30 and 32 having the same initial diameterand wall thickness as workpiece 10 before the swaging operationsoccurred. Portions 30 and 32 are thereafter separated by cutting along aplane centered therebetween as indicated by reference numeral 34 inFIGURE 3, after which either of the individual portions may be used toform an article of desired final shape such as the hollow spheresuggested by FIGURE 4.

Referring to FIGURE 4, workpiece portion 32 having swaged end portions26 and 28 is placed within a hollow die 40 which may be separatelyformed in two oppositely corresponding half sections held together bysuitable means such as a plurality of elongate bolts 42 and nuts 44. Die40 has a hollow cavity 46 therein, the cavity walls comprising aworkface 48 size and shaped according to the desired final configurationof the article sought to be made from workpiece 32. Cavity 46 hascircular openings 50 and 52 at the top and bottom thereof, respectively,sized to receive end portions 26 and 28 and to make substantiallycontinuous surface contact therewith but not necessarily inforce-fitting relationship. Die 40 is supported on a substantiallyplanar base plate 54 which provides a stable reference surface for thedie and for a vacuum chamber 56 of convenient form such as a cubical orcylindrical shape and which is sized so as to enclose die 40 therewithinand to provide a spacial gap 58 between chamber 56 and die 40.

Chamber 56 is preferably of high strength material such as steel and isprovided with a sealing flange 60 which is coextensive with the loweredge of the vacuum chamber and is adapted to overlie a groove 62 in thesurface of steel base plate 54 as shown in FIGURE 4. Groove 62 containsa suitably resilient sealing element 64 which may comprise a rubber ringof familiar type and round in cross section but deformable undercompression within groove 62 as shown in FIGURE 4. Chamber 56 is furtherprovided with a vacuum line 66 for connection with a pump (not shown)whereby all atmosphere or other gases within chamber 56 may be evacuatedthrough vacuum line 66. To provide escape means for excess explosiveforce and water or other filler materials during the explosive formingprocess, a blasting exit 68 is provided in vacuum chamber 56 andsubstantially aligned over top portion 26 and opening 50 of die 40 asshown in FIGURE 4. Blasting exit 68 may be substantially circular inshape and a cover member 70 of suitable material, such as hard rubber orplastic having suflicient rigidity to resist the force of the vacuumwithin chamber 56, is adapted to cover exit 68 in sealing relationshiptherewith. Restraining means of any appropriate form which mayillustratively comprise I beam sections 72 and 74 may be provided in therelationship shown by FIGURE 4 to inhibit or otherwise restrain theamount of movement of cover member 70 under the force of detonationduring the explosive forming process.

The explosive forming process maybe'gin by filling workpiece portion 32with a suitable filler such as water and immersing an explosive charge78 substantially in the center thereof and appropriately connected fordetonation by electrical means in a conventional manner. With workpiece32 positioned in die 40 as seen in FIGURE 4, chamber 56 is evacuated byconnection of vacuum line 66 with a vacuum pump, whereby cavity 46 indie 40 is also simultaneously evacuated along with spacial gap 58 due topressure communication means interconnecting the same such as passage'80 unless communication between the mentioned areas is already providedby looseness of fit between workpiece portion 26 and opening 50, forexample. Thereafter, explosive charge 78 is detonated, resulting inexplosive force in the form of one or more shock waves radiatingoutwardly from charge 78 and causing deformation of workpiece portion 32outwardly and forcefully into contact with Workface 48 of die cavity 46.During the outward deformation of the mid portion of workpiece 32,elongation and resultant thinout of the material in the mid portion isless than it would have been had a straight tube been bulged having thediameter of ends 26 and 28. Local thinning of walls in portions 26 and28 due to the stated pattern of expansion within die 40 is avoided bythe initially heavy wall thickness in both said portions achieved by theswaging step described above in connection with FIGURES l and 2. Suchavoidance permits articles of greater than critical length (with respectto the amount of diametral expansion) to be formed by the processdisclosed herein.

From the description set forth above and the structure shown in thedrawings, it will be understood that the inventive method disclosedherein essentially comprises at least two successive deformation stepswhich include the initial compression of a workpiece blank to increasethe wall thickness thereof at desired predetermined locations, followedby expansion of the same workpiece at other locations thereon to achievea final workpiece shape in the finished article. Moreover, it will beunderstood that either of the successive steps thus illustratively shownmay be separately used to achieve different or other workpiece shapes orused in conjunction with other methods and deformation steps known tothe explosive forming art. Thus, the use of vacuum chamber 56 toevacuate cavity 46 in die 40 affords particular advantages in the methoddescribed in connection with FIGURE 4, by avoiding the problemsassociated with sealing of a die cavity such as would be requiredbetween end portions 26, 28 and their respective openings '50 and 52 inthe absence of vacuum chamber 56. The ease, economy and rapidity of theevacuating method thus recited is a separate and very significantfeature of the invention disclosed herein. Conversely, the successivesteps of swaging and then expanding tubular workpiece shell blank asdiscussed above may be advantageously practiced without the use ofvacuum chamber 56. In addition, depending on workpiece materialcharacteristics and amount of explosive force required to deform it,filler materials 12 and 76 may be omitted when not required in a givencase, and expansion of blank 32 mayconceivably be accomplished bydieless forming in which case die 40 might be eliminated as unnecessaryin practicing the broad concept taught herein.

While the particular structural details and method set forth above andin the drawings are fully capable of attaining the objects and providingthe advantages herein stated, the structure and method thus disclosedare merely illustrative and could be varied or modified to produce thesame result or portions of the process separately used without departingfrom the scope of the inventive concept as defined in the appendedclaims.

We claim:

1. A method of forming a throat section in a rigid elongate cylindricalblank having a substantially constant initial cross-sectional area alongsome portion of its length, said method comprising:

initially preplacing firmly rigid but crushable filler materialcomprising low density plastic material within said blank, placing aprotective sleeve of flexible force-transmitting materialcircumferentially around said blank and in contact with the outersurface thereof, arranging explosive material in the form of a pluralityof layers in sheet form wrapped over said protective sleeve and insubstantial axial alignment therewith, and

detonating said explosive sheet material to apply substantially uniformcircumferentially distributed radially inwardly directed force to saidblank.

2. A method of forming a bulbous article from an elongate hollow tubularblank having a mid-portion of larger diameter than each of the two endportions adjoining said mid-portion, said method comprising:

supporting said blank by restraining said two ends in a support,

placing an explosive charge with said blank,

filling said blank with shock-wave transmitting material,

placing said blank and said support in a surrounding vacuum chamber,evacuating said chamber to lower the pressure therein substantiallybelow atmospheric pressure, and

detonating said explosive charge to apply substantially uniform forceradially outwardly in an amount sufficient to deform said tubular blankinto said bulbous shape.

3. A method of forming a bulbous article from an elongate hollow tubularblank having a mid-portion of larger diameter than each of the two endportions adjoining said mid-portion, said method comprising:

supporting said blank in a surrounding die having a hollow cavitytherein,

placing an explosive charge Within said blank,

filling said blank with shock-wave transmitting material,

placing said die into a vacuum chamber,

evacuating said chamber to lower the pressure within said die cavitysubstantially below atmospheric pressure, and

detonating said explosive charge to apply substantially uniform forceradially outwardly in an amount sufficient to deform said tubular blankinto said bulbous shape.

4. A method of forming a bulbous article by explosive force applied to ahollow workpiece, said method comprising:

placing said workpiece in a surrounding die having a cavity therein,

placing said die within a surrounding vacuum chamber,

placing an explosive charge in spaced relationship from said workpiece,

evacuating said vacuum chamber to lower the pressure in said die cavitysubstantially below atmospheric pressure, and

detonating said explosive charge to apply force to said workpieceanddeform the same.

5. A method of forming an article from an elongate hollow tubular blankcomprising the steps of:

placing said tubular blank in a surrounding die having a workface spacedapart from said workpiece,

placing said die in a vacuum chamber,

placing an explosive charge within said blank,

filling said blank with shock-wave transmitting material,

evacuating said chamber to lower the pressure in said die substantiallybelow atmospheric pressure, and

detonating said explosive charge with suflicient force to deform saidblank into contact with said work face.

6. A method of forming a bulbous article from an elongate hollow tubularblank having a substantially constant initial cross-sectional area alongsome portion of its length, said method comprising:

' 7. The method set forth in claim 6 above, further including:

prior to said final detonating step, placing said blank in a vacuumchamber, and

evacuating said chamber to lower the pressure therein substantiallybelow atmospheric pressure.

8. The method set forth in claim 6 above, further including:

prior to said final detonating step, placing said blank in a die havinga cavity therein, and

filling said blank with shock wave transmitting material.

9. A method of forming a bulbous article from an elongate substantiallycylindrical rigid blank, said method comprising:

placing rigid but crushable filler material within said blank,

placing a protective sleeve of flexible force-transmitting materialcircumferentially around said blank at a first axial location thereon,

placing an explosive material circumferentially symmetrical around saidprotective sleeve and substantially aligned axially therewith,

detonating said explosive material to apply substantially uniformcircumferential force to said blank in a radially inward direction todecrease the cross-sectional size of said blank and increase the wallthickness of said blank at said location,

repeating all the above steps at a second axial location on said blank,said second location being spaced apart from said first location,

placing said blank in a die having a hollow cavity and adapted tosupport said blank at said first and second locations and to leave theportion of said blank between said spaced apart first and secondlocations unsupported within said hollow cavity,

filling said blank with shock wave transmitting material,

placing an explosive charge within said blank,

placing said die within a vacuum chamber,

8' evacuating said chamber to lower the pressure in said die cavitysubstantially below atmospheric pressure, and detonating said explosivecharge to deform said blank outwardly in said unsupported portion. 10.In apparatus for explosively forming an article by deformation of afluid-containing workpiece:

die means including a die for surrounding said workpiece,

platform means for supporting said die,

vacuum chamber means comprising an enclosure for containing said die toisolate said die from surrounding atmosphere, said enclosure beingsupported on said platform means, and

seal means for sealing said vacuum chamber means to said platform meansin pressure-tight relationship.

11. The apparatus set forth in claim 10 above, wherein:

said enclosure comprises a substantially cylindrically 'shaped chamberhaving an annular flange thereon,

and

said seal means comprises an annular groove in said platform means and aflexible ring contained within said groove and adapted to besubstantially uniform- V ly compressed within said groove by saidflange.

12. A method of forming a throat section in a rigid elongate cylindricalblank having a substantially constant initial cross-sectional area alongsome portion of its length, said method comprising:

placing an explosive material circumferentially symmetrical about theouter surface of said blank at a location along said portion, and

detonating said explosive material to apply substantially unifor-mcircumferentially distributed radially inwardly directed force to saidblank to reduce the cross-sectional area thereof to a minimum at saidlocation whereby said initial cross-sectional area v progressivelydecreases on either side of said location toward said minimumcross-sectional area,

initially preplacing firmly rigid but crushable filler material withinsaid blank prior to said detonating step,

placing a protective sleeve of flexible force-transmitting materialcircumferentially around said blank and in contact with the outersurface thereof prior to placement of said explosive material, and

arranging said explosive material in substantial alignment axially oversaid sleeve, and

restraining both ends of said elongate cylindrical blank to preventmovement of said ends during said detonating step.

References Cited UNITED STATES PATENTS 2,367,206 1/ 1945 Davis 29-4212,961,357 11/1960 Earnhardt et al 7256 2,983,242 5/1961 Cole 72563,068,822 12/1962 Orr et al 7256 3,112,166 11/1963 Montgomery et al 72563,120,827 2/ 1964 Abegg et al 7256 3,160,949 12/1964 Bussey et al 29-4213,186,203 6/1965 Brady et a1. 7256 3,242,939 3/1966 Fogg 29-4213,247,581 4/1966 Pellizzari 29-157 3,364,708 1/ 1968 Padberg 7256RICHARD J. HERBST, Primary Examiner

